The present invention relates to an image forming apparatus which forms an image by an electrophotographic method, and more particularly to an improvement of a fixing device of the image forming apparatus.
(1) A technology for energy saving is one of the subjects of development in the technical field relating to the image forming apparatus which forms an image on a recording material by the electrophotographic method. The power consumption of the electrophotographic image forming apparatus depends upon the power consumption of the fixing device so largely that the energy saving in the fixing device represents the saving of power consumption in the image forming apparatus, and accordingly, the development of the technology for suppressing the energy consumption in the fixing device is the aforementioned subject of development.
Concerning the electric power which is consumed in the fixing device, the energy consumption under a stand-by condition is overwhelmingly greater than the energy consumption under an image forming condition. Accordingly, there is paid much attention for suppressing the energy consumption of the fixing device in the stand-by condition, that is, there is paid much attention to the development of the fixing device, which is kept under the condition that the power supply is not given to a heat source of the fixing device, or the condition that lower electric power is given even if power is given, and which can rise to the condition being capable of fixing within a short time, when a starting button of an image formation is operated, or when an instruction for an image formation is given from the outside.
A belt having small heat capacity is influential for a heating member of the fixing device having a short rise time mentioned above, and hitherto, there have been a great number of patent applications concerning the fixing device in which the belt is used for the heating member.
Further, it is performed that temperature of the heating member is raised to the temperature capable of fixing, while the heating member is released from a pressure applying member. Since the aforementioned rise-up of the heating member prevents heat of the heating member from traveling to the pressure applying member, the heat capacity of a heat receiving system becomes so small that the heating member can rise to be the temperature level capable of fixing within a short time.
(2) Generally, the fixing device is provided with the heating member and the pressure applying member. The fixing device fixes the toner image on the recording member by heat and pressure, by making both of the heating member and the pressure applying member to come into contact with each other by the prescribed pressure, and making a recording member having an unfixed toner image to pass between the heating member and the pressure applying member.
In the conventional fixing device, the pressure applying member is provided under the condition where the pressure applying member is brought into contact with the unmovable heating member, or is released from the unmovable heating member, and the pressure applying member is pressed to the heating member by the movement from the released position that is not under the acting condition, when the image formation is performed. Further, when releasing the pressure applying member from the heating member, the pressure applying member has been moved against urging of an urging means.
(1) It has become clear that if there is a difference between the surface speed of the heating member and that of the pressure applying member, when the pressure applying member is brought into contact with the heating member, the difference causes stress which gives undesirable influence upon these members. That is, it has become clear that the surface of the heating member or the pressure applying member changes in formation or changes in quality. Especially, when one having a rubber surface with lower hardness on the surface or a belt is used as the heating member, these changing are clear, and off-set occurs or the belt is broken in an extreme case.
The object of the present invention is to solve the above-mentioned problem of the fixing device wherein the pressure applying member is kept to be released from the heating member during the stand-by condition, and the pressure applying member comes in contact with the heating member when the image is formed, and further, to provide a fixing device of an energy saving type which can keep good fixing performance for a long time, still further, to provide an image forming apparatus having therein the above-mentioned fixing device.
(2) Under the construction that the pressure applying member is brought into pressure contact with the heating member by the urging means, the pressure applying member is brought into contact with the heating member by the prescribed pressure when pressure is applied. Therefore, it is necessary to apply releasing power which is stronger than pressuring power on the pressure applying member, when the pressure applying member is released from the heating member, which means that great power is necessary for releasing the pressure contact. A motor is used generally as a driving means which performs pressure contact/releasing of pressure contact of the pressure applying member, however, the motor having large power is necessary, resulting in problems that the electric power consumption is large and the cost is high.
Another object of the invention is to solve the above-mentioned problems of a mechanism which performs pressure contact/releasing of the pressure applying member onto the heating member, and to provide a fixing device of a type of low energy consumption and low cost, and to provide an image forming apparatus having therein the above-mentioned fixing device.
The objects of the invention will be attained by either one of the Structures shown below.
Structure (1) The fixing device in which a pressure applying member is provided to be in contact with or away from a heating member which heats a toner image, and fixing is conducted by making the recording material carrying thereon a toner image to pass between the heating member and the pressure applying member, under the condition that the pressure applying member is brought into pressure contact with the heating member, wherein there are provided a changeover means which switches a condition of the heating member and the pressure applying member between a pressure contact condition and a pressure contact released condition, and a driving means which drives the heating member and the pressure applying member under the pressure released condition, and when the changeover means changes the condition from the pressure released condition to the pressure contact condition, the changeover means conducts switching so that either one of the heating member and the pressure applying member may be separated from the driving means, and may touch the other party being driven by the driving means under the condition that either one of the heating member and the pressure applying member released from the driving means is moved by inertial force.
Structure (2) The fixing device mentioned in the Structure (1), wherein there is provided the driving means which transports the recording material by driving the heating member.
Structure (3) The fixing device mentioned in the Structure (1), wherein the changeover means forms the pressure contact condition and the pressure released condition, by changing the position of the pressure applying member.
Structure (4) The fixing device mentioned in either one of the Structures (1) to (3), wherein the changeover means uncouples the pressure applying member from the driving member.
Structure (5) The fixing device mentioned in either one of the Structures (1) to (4), wherein the heating member is represented by a heating belt, and the fixing is performed by making the heating belt to touch the recording material.
Structure (6) The fixing device mentioned in the Structure (5), wherein there is provided a heating means which heats the heating member.
Structure (7) The fixing device mentioned in the Structure (6), wherein the heating means has a heat source and a heating roller which is heated by the heat source, and about which the heating belt is trained.
Structure (8) The fixing device mentioned in either one of the Structures (5) to (7), wherein the heating belt is provided with a base body and a heat-resistant elastic layer formed on the base body.
Structure (9) The fixing device mentioned in either one of the Structures (1) to (4), wherein each of the heating member and pressure applying member is composed of roller.
Structure (10) The fixing device in which a pressure applying member is provided to be in contact with or away from a heating member which has a toner image, and fixing is conducted by making the recording material carrying thereon a toner image to pass between the heating member and the pressure applying member, under the condition that the pressure applying member is brought into pressure contact with the heating member, wherein, there is provided a driving means which drives the heating member and pressure applying member under the condition that the pressure applying member is released from the heating member, and the driving means drives either one of the heating member and the pressure applying member through a torque limiter.
Structure (11) The fixing device mentioned in the Structure (10), wherein the torque limiter has transmission torque Q having the range shown by the following formula:
19.6xc3x9710xe2x88x924 less than Q less than 9.8xc3x9710xe2x88x923(Nxc2x7m). 
Structure (12) The fixing device mentioned in the Structure (10) or (11), wherein the heating member is composed of a heating roller.
Structure (13) The fixing device mentioned in the Structure (10) or (11), wherein the heating member is composed of the heating belt.
Structure (14) The fixing device having therein the heating member for heating a toner image, the pressure applying member arranged to face the heating member, an urging means which urges the pressure applying means, and a changeover means which changes the condition of the pressure applying member from the pressure contact condition to the heating member to the pressure contact released condition by controlling the urging means, wherein urging power by the urging means under the pressure contact released condition is lower than that under the pressure contact condition.
Structure (15) The fixing device mentioned in the Structure (14), wherein the urging means forms a non-urging condition under the pressure released condition.
Structure (16) The fixing device mentioned in the Structure (14), wherein the urging means forms an urging condition under the pressure released condition.
Structure (17) An image forming apparatus wherein there are provided an image forming means which forms an unfixed toner image on the recording material, and the fixing device mentioned in either one of the Structures (1) to (16).